A MAPK Cascade Downstream Of Receptor RLK7 To Regulate Growth And Disease Resistance In Arabidopsis Thaliana

The Mitogen-Activated Protein Kinase（MAPK）signal cascade,which composed of MAPKs,MAPK kinases（MAPKKs）and MAPKK kinases（MAPKKKs）,functions in signal transduction from membrane receptors into nucleus.The three-layered MAPK cascades are important for the signal transduction in plant development and environmental stress response through protein phosphorylation.Among those,the MKK4/MKK5-MPK3/MPK6 cascade reported involve in the regulation of plant growth,development,and disease resistance.The plasma membrane localized receptor RLK7（Leucine-rich repeat receptor-like kinase7）recognize the PAMP（Pathogen/microbe-associated molecular pattern）-induced secreted peptides（prePIPs）,and induces downstream immune responses in plants.In this study,we reported the role of MAPK cascade in PIPs-RLK7 mediated signaling in root development and immunity.1.The MKK4/MKK5-MPK3/MPK6 cascade is required for the PIPL3-RLK7 mediated root growth inhibition.Previous reports showed that PIPL3（PAMP-induced secreted peptide-like 3）inhibits later root（LR）and LR primordia（LRP）development in roots.Here we show that the PIPL3-mediated LR and LRP growth suppression was abolished in rlk7 mutant.Biochemical analysis showed that PIPL3 induces phosphorylation of MPK3/MPK6.By using the conditional loss-of-function double mutants of MPK3/MPK6,MPK3SR and MPK6SR,and mkk4 mkk5 double mutant,we observed that the PIPL3-mediated LR and LRP suppression was also abolished.These results suggested that the MKK4/MKK5-MPK3/MPK6 signaling cascade is required for the PIPL3-RLK7 mediated LR and LRP regulation process.Furthermore,the wrky30 mutant also exhibits similar phenotype with rlk7,which are insensitive to the PIPL3-mediated LR suppression.Moreover,DEX induced expression of active form of MKK4(MKK4^DD)leads to the upregulation of WRKY30.These results illustrating that WRKY30 may function downstream of MKK4/MKK5-MPK3/MPK6 cascade in the regulation of LR and LRP development.2.The MKK4/MKK5-MPK3/MPK6 cascade participates in the process of PIPs-RLK7 synergistic amplification of flg22-induced immunity.Perception of flg22 triggers repid activation of innate immune responses,such as elevation of cytosolic calcium,generation of reactive oxygen species（ROS）,phosphorylation of MAPKs,callose deposition,and transcriptional reprogramming of the cell,and leading to the enhanced resistance against pathogens.RT-qPCR data showed that the expression of prePIPs was significantly up-regulated in MKK4^DD after DEX induction.Meanwhile,comparing with Col-0,flg22-induced prePIPs expression was significantly reduced in MPK3SR and MPK6SR,suggesting MKK4/MKK5-MPK3/MPK6 cascade regulates the expression of prePIPs.We also found that pretreatment flg22 significantly reduces the PstAvrRpt triggered hypersensitive response（HR）in Arabidopsis.This flg22-indueced HR suppression was reduced in rlk7 mutant,suggesting that PIPs-RLK7 may involve in this process.Pretreatment of PIPL3 also leading to the suppression of PstAvrRpt2-induced HR through RLK7.These results indicating that PIPL3-RLK7 may contribute to flg22-triggered immunity synergistically.